Using Or mutably - query violates a unique borrow

[junglie85]

I want to mutable query for entities which have 2 possible sets of components, like so:

for (_id, r) in world.query_mut::<Or<
        (&mut Transform, &Physics, &Drawable, &Input),
        (&mut Transform, &Physics, &Drawable),
    >>() {

The only difference is that one has an Input component.

However, when I run this, I get the following error:

thread 'main' panicked at 'query violates a unique borrow', C:\Users\junglie85\.cargo\registry\src\github.com-1ecc6299db9ec823\hecs-0.9.0\src\query.rs:848:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\examples\sandbox.exe` (exit code: 101)

Is what I'm trying to do possible and I'm just misunderstanding the API, or is it not possible and I need to do 2 separate queries?

[Ralith]

This should work, but it looks like there's a bug here; thanks for the report! In the mean time, what you probably want to do is use the query (&mut Transform, &Physics, &Drawable, Option<&Input>) anyway, which will definitely work.

[Ralith]

On closer review, this is working as intended. Or is documented as:

Holds an L, or an R, or both

When an entity satisfies both, you get both. Two &mut Transforms to the same value would be illegal aliasing. If both clasues of an Or contain the same &mut T, you need to ensure that no entity will ever match both. In your case you should use an Option instead, so that the common references aren't needlessly duplicated.

[junglie85]

Ah, you're very right re Or. I understand now why the error is being raised - my query satisfies both L and R so we have two mutable borrows of the same data, which cannot happen.

I originally tried the &Option<Input> and it didn't work, which is why I reached for Or. Thanks for your example - I had the borrow in the wrong place!